It is known to add certain resins to paper, usually during the paper-making process, to improve wet and/or dry-strength of paper. The type of resin added depends on the properties desired in the final paper product. For tissue, toweling and certain other applications, it is desirable that a strengthening resin be added to the paper in order to impart dry and temporary wet-strength.
In commercial and industrial towel markets and tissue markets there is a need for wet-strength resins which have high initial wet-strength and a large and fast decay rate (i.e., temporary wet-strength). It is desirable that papers containing temporary wet-strength resins lose much of their strength on prolonged immersion in water such as on disposal in sanitary systems. It is also desirable that broke containing temporary wet-strength resins be easily reprocessed into paper during the papermaking process.
Resins have been based upon polyacrylamide/glyoxal resins which have been found to impart wet and dry-strength to paper and paperboard. These resins impart moderate wet-strength to paper however, the wet-strength decay has been found to be inadequate. Paper containing those resins retain too much of their initial wet-strength over time when exposed to water, see U.S. Pat. No. 3,556,932 to Coscia et al. and U.S. Pat. No. 3,709,857 to Faessinger.
Other resins have been based on starch based products which contain a latent aldehyde functionality. These resins while exhibiting good wet-strength decay, exhibit only low initial wet-strength, see U.S. Pat. No. 4,866,151 and EP 0 418 917 A2.
Another approach for obtaining temporary wet-strength resins involves modifying standard cationic polyacrylamide/glyoxal resin through the inclusion of a non-nucleophilic comonomer (e.g., dimethylacrylamide), see U.S. Pat. No. 4,603,176. Still another approach involves the use of temporary wet-strength resins with nitrogen heterocyclic, non-nucleophilic functionalities (e.g., N-vinylpyrrolidinone (NVP)), see U.S. Pat. Nos. 4,981,557, 5,008,344, 5,138,002 and 5,085,736.
Another approach for obtaining temporary wet-strength resins has been through the use of wet-strength resins containing latent aldehyde functionalities. One approach involved the reaction of a cationic polyacrylamide with dimethoxyethanal instead of glyoxal. These resins were found to give comparable decay to polyacrylamide/glyoxal resins which exhibit inadequate temporary wet-strength characteristics, see U.S. Pat. Nos. 5,401,801 and 5,490,904.
Also, it is well known to add certain resins to paper, usually during the paper-making process, to improve the dry-strength of the resultant paper. It is also well known that certain dry-strength additives result in an increase in a paper's wet-strength and visa versa. However, it is not always desirable that paper with increased dry-strength also exhibit an increased wet-strength since increasing a paper's wet-strength makes paper more difficult to repulp. Difficulties exist when reclaiming or recycling paper back to its individual fibers when conventional resins have been used to impart strength to the paper. Achieving de-fibering of these papers involves a process of exposing the papers to sufficient heat and under proper chemical conditions to initiate or maintain amide hydrolysis, while subjecting the papers to forces for breaking apart fiber networks contained in the papers without significantly damaging the fibers themselves.
Many polymers that improve a paper's dry-strength are anionic under normal papermaking conditions, e.g., sodium carboxymethylcellulose, carboxymethyl guar, and copolymers of acrylamide and acrylic acid or sodium acrylate. A cationic compound may be used to retain these anionic substances on the pulp fibers which are themselves anionic by nature. These cationic compounds may be wet-strength agents such as polyaminoamide-epichlorohydrin resins, highly cationic polymers such as poly(diallyldimethylammonium chloride), polymers of dimethylamine and epichlorohydrin, modified starches, aluminum compounds, etc.
In U.S. Pat. No. 5,338,406 to Smith, a dry-strength system for a "water-soluble, linear, high molecular weight, low charge density cationic polymer having a reduced specific viscosity greater than two deciliters per gram (&gt;2 dl/g) and a charge density of 0.2 to 4 milliequivalents per gram" with "at least one water-soluble, anionic polymer having a charge density less than 5 meq/g" is disclosed. The polyelectrolyte complex of Smith is useful as an additive for providing dry-strength to all types of paper, particularly for those papers which are produced using unbleached pulp.
In U.S. Pat. No. 5,338,407 to Dasgupta, a process for enhancement of paper dry-strength without reducing its softness is disclosed which comprises adding a mixture of an anionic carboxymethyl guar, carboxymethyl bean gum or carboxymethyl hydroxyethyl guar with various cationic additives to a bleached pulp furnish. The cationic additive may be a polyarnide-epichlorohydrin resin and, if the cationic additive is a wet-strength resin, the enhancement of the paper's dry-strength without reducing its softness is achieved. Additionally, the wet-strength of the paper is increased.
In U.S. Pat. No. 5,543,446 to Rodriguez, a water-soluble terpolymer for use as a dry-strength additive for paper is disclosed. The terpolymer comprises a reaction product of monomers consisting essentially of acrylamide or methacrylamide, an ethylenically unsaturated aliphatic carboxylic acid or a salt thereof and a water-soluble polyvinyl monomer. The resulting dry-strength additive is water-soluble for convenience of operation and readily dilutable with water and is easily miscible with papermaking fibers.
In Canadian Patent No. 1,110,019, "a process for manufacturing paper having improved dry-strength which comprises mixing an essentially alum-free pulp slurry with a water-soluble cationic polymer and subsequently adding a water-soluble anionic polymer to the essentially alum-free pulp slurry" is disclosed.
In addition to the above, polyamidoamine-epichlorohydrin resins have been used extensively as wet-strength agents for paper. Paper treated with these resins do not exhibit temporary wet-strength characteristics. These resins are typically prepared in a two-step process. In the first step, a polyamidoamine prepolymer is prepared from a diacid (e.g., adipic acid) and a polyamine (e.g., diethylenetriamine). Then in the second step, the prepolymer is reacted with epichlorohydrin in an amount equal to or greater than the amount of secondary amine groups in the prepolymer. In a subsequent step, a small amount of epichlorohydrin reacts to effect branching of the prepolymer, accompanied by an increase in molecular weight. However, a majority of the epichlorohydrin reacts with the prepolymer to give reactive functional groups, specifically, either aminochlorohydrin or azetidinium. It is well known to those skilled in the art of papermaking that the above-described cationic wet-strength resins may be used in combination with anionic acrylamides or anionic cellulose derivatives. However, papers containing these combinations exhibit increased wet-strength as well as increased dry-strength.
Therefore, the need remains for temporary wet-strength resins for use in paper which provide the paper with sufficient initial wet-strength and which exhibit a large and fast decay rate.
The need also remains for dry-strength resins for use in paper which provide paper with sufficient dry-strength and which exhibit temporary wet-strength thereby permitting paper containing the resins to be easily repulped.